What is the smallest fluid deposit size I can achieve with a dispense tip?

Normally, the minimum deposit size achievable by a dispensing tip is typically twice that of the tip’s inner diameter (ID). That is just an approximation, however. The four factors that can affect fluid deposit size are:

• Dispense tip diameter
• Fluid viscosity
• Properties of the surface being dispensed onto (high-viscosity fluids only)
• Dispenser pulse duration

Figure 1: Dispensing tips are available in a range of diameters, form factors, and materials designed to support precision dispensing for a variety of application parameters.

How does dispense tip diameter affect minimum fluid deposit size?

The diameter of the deposit will be larger than the ID of the tip. Fluid spreads as it leaves the tip, so a tip with an ID of 0.004 in, for example, will produce a deposit wider than 0.004 in. The rule of thumb is to choose a dispense tip with an ID that is half the diameter of the desired deposit.

The exception to this rule is particle-filled materials. Depending on the concentration of particles, the fluid could clog the dispensing tip. A good starting point is to choose a dispensing tip with an ID that is 10 times the size of the particle in the fluid. For example, if the particle size is 20 µm, choose a tip with an ID of 200 µm. Be sure to test the system and understand that for very high concentrations, multiplying by 10 may not be enough.

How does fluid viscosity affect minimum fluid deposit size?

Thin fluids and cyanoacrylates

The rule of thumb mentioned above is an approximation. Thin fluids and cyanoacrylates may spread significantly more. Start by specifying a dispense tip with a 2:1 ratio of desired deposit diameter to dispense tip ID. Test and change the tip size as required.

Thick pastes and particle-filled materials

Ultrahigh viscosity fluids present different challenges. Extremely narrow cannulas can create too much back pressure, making it impossible to force the material through the tip, even at very high pressures. As a result, some materials have a minimum dispensing tip ID that may be larger than the minimum tip size we offer.

How do properties of a surface affect minimum fluid deposit size?

High viscosity fluids may not release easily from the dispensing tip. In these cases, interaction with the surface can significantly affect the dispensing process.

Distance between dispense tip and surface at deposition

To ensure release, the tip must either come into contact with the surface or be close enough that the fluid will contact and adhere to the surface.

Surface roughness

The rougher the surface, the higher the level of friction, and thus, adhesion. This is particularly important for deposition of thick pastes at high repetition rates.

How does pulse duration affect minimum fluid deposit size?

Pneumatic dispensers control precision dispensing by producing air pulses. Each pulse puts pressure on the fluid column, releasing material from the dispensing tip. These dispensers are fed by compressed air and use a standard pneumatic three-way valve to control pulse duration. The shorter the pulse, the smaller the fluid deposit.

That’s the theory but there is some fine print. That internal valve is a solenoid that requires time to open. As a result, it is the solenoid that determines the minimum pulse duration, not the setting on the display. Setting a dispenser for a pulse duration of 0.001 s will do nothing if the solenoid requires 0.008 s to go through full stroke. In fact, it’s best to avoid setting the dispenser for the minimum solenoid stroke time. The most effective approach is to select a duration that allows adjustment backward and forward to optimize the process.

To achieve your goals, start by specifying a tip with an ID half the size of the required diameter. If that doesn’t work, experiment with changes to positioning, pulse duration, pressure, etc. Also, remember that not every requirement can be realized. When in doubt, reach out to your Nordson EFD application engineering team for assistance in achieving the best performance for your process, contact us.